This invention relates to a method of growing low-dislocation-density material atop a substrate that is sufficiently lattice mismatched to the material that numerous threading dislocations normally form during the growth process or during the cool-down after growth. An example where such a problem is encountered is Group III nitride materials.
The growth of many materials, including Group III nitride-based materials, relies on heteroepitaxy on substrates that often have appreciably different lattice constants and thermal expansion coefficients than the material being grown upon it. This can lead to a very high density of dislocations in the resulting films. In the case of Group III nitride materials, high densities of dislocations degrade the electronic and optical properties of the nitride materials and degrade the performance of devices made therefrom. Hence, it would be highly desirable to have an easy-to-implement way to reduce the dislocation density in heteroepitaxially grown layers.
An and coworkers have reported maskless heteroepitaxial growth of GaN on a Si substrate with a self-assembled sub-micrometer-sized silica-ball layer (S. J. An, Y. J. Hong, G.-C. Yi, Y.-J. Kim, and C. K. Lee, “Heteroepitaxial Growth of High-Quality GaN Thin Films on Si Substrates Coated with Se-lf-Assembled Sub-micrometer-sized Silica Balls,” Adv. Mater. vol. 18 (2006) pp. 2833-2836). In the method of An, prior to the GaN layer growth, AlN buffer layers were deposited on the silica-ball-coated substrates in order to prevent meltback etching. The silica balls had been placed on the substrate prior to AlN growth to form an SBS substrate. Prior to the growth of the AlN buffer layer on the SBS substrates, oxide layers on the Si substrate surfaces were removed by annealing under a hydrogen flow at high temperature (1100° C.). An 80 nm AlN buffer layer with a few monolayers of predeposited Al was deposited on the SBS substrates, and GaN films were grown on the AlN buffer layers.
Ueda and coworkers report a modification of the method of An (K. Ueda, Y. Tsuchida, N. Hagura, F. Iskandar, K. Okuyama, and Y. Endo, “High performance of GaN thin films grown on sapphire substrates coated with a silica-submicron-sphere monolayer film. Ueda employs sapphire substrates, which are used because the transparent properties of sapphire substrates are superior to those of Si substrates. First, silica submonolayer films were deposited on the sapphire substrates using a spin-coating method. Ueda states that the spin-coating method has practical advantages in terms of production rate and uniformity compared with other sol-gel methods, such as dip coating, wet coating, and the Langmuir-Blodgett. In the method of Ueda, sapphire substrates were cleaned with buffered hydrofluoric acid. The silica colloid suspension was dropped onto the center of the substrates when spinning is begun. The silica balls were on the sapphire substrate before the first GaN growth. GaN films were grown on the sapphire substrates coated with silica spheres using MOCVD. A low-temperature GaN buffer layer was grown at 485° C. and 1 atmosphere, an undoped AlGaN layer (at 800° C.) and undoped GaN layer (at 900° C.). Susceptor temperature and pressure were changed to 1040° C. and 0.25 atmosphere to grow 5-micron-thick undoped GaN films. SEM images show that the silica-sphere-monolayer is located below the GaN film. The nonmonodispersity of the silica particles evident in the SEM images results from random distribution of the spheres during the spin-coating process (i.e., the spheres are not arrange in a line). Dislocations appear directly above the silica spheres. Ueda states that their results suggest that the nanostructures silica layer strongly promoted epitaxial lateral overgrowth of GaN and reduced dislocation. Increasing surface coverage of silica spheres reduced dislocations in the GaN epilayer, although reduction of GaN dislocations was limited near silica spheres. Coating the substrate with a silica-sphere monolayer prior to GaN growth increased the output optical power of LED samples by as much as 2.5 fold. The threading dislocations in the sample illustrated in Ueda's transmission electron micrograph (TEM) FIG. 3b is on the order of 109/cm2. This is of the same order as can be obtained for GaN growth directly on sapphire.